Method of producing phthalic anhydrides



Nov. 14. 1967 H.1 RILEY ET AL 3,352,887

METHOD OF PRODUCING PHTHALIC AiJHYDRIDE Original Filed Jan. 5, 1961 2SheetS-Sheet 1 I l V I l I l I I H 1 2 .5 4 saves/ 2o RAT/0 K 0; v 0

W -fl' TIME- MINUTES INVENTORS Harry Lister Riley Andrzejlfomansk BY I 'ATTORffEYJ' Nov. 14, 1967 H. L. RILEY ET AL METHOD OF PRODUCING PHTHALICANHYDRIDE Original Filed Jan. 5, 1961 2 Sheets$heet 2 INVENTORJ m nn Wam A 1L0 M w n L n Z n #fi United States Patent 3,352,887 METHOD OFPRODUCING PHTHALIC ANHYDRIDES Harry Lister Riley, Oxford, and AndrzejRomanski, Sheffield, England, assignors to United Coke and ChemicalsCompany Limited, Rotherham, England Original application Jan. 3, 1961,Ser. No. 79,971, new Patent No. 3,226,338, dated Dec. 28, 1965. Dividedand this application Sept. 16, 1965, Ser. No. 487,828 Claims priority,application Great Britain, July 3, 1959, 22,973/59 The portion of theterm of the patent subsequent to Dec. 28, 1982, has been disclaimed 1Claim. (Cl. 260-3464) ABSTRACT OF THE DISCLOSURE A catalyst composed ofa glass formed from a mixture of vanadium pentoxide and potassiumpyrosulfate and carried on silica gel particles is used in a fluidizedbed to convert naphthalene vapor and air into phthalic anhydride.

This application is a division of our copending application Ser. No.79,971, filed Ian. 3, 1961, now Patent No. 3,226,338 which in turn is acontinuation-in-part of our copending application Ser. No. 38,858, filedJune 27, 1960 for Manufacture of Catalysts, now abandoned.

Many catalytic reactions in the vapor phase can advantageously becarried on with the catalyst in the form of particles fluidized by thereacting gases or vapors. Catalysts containing vanadium and potassiumsulphate are used in this form in, for example, the oxidation ofnaphthalene to phthalic anhydride.

In the manufacture of catalysts containing vanadium and potassiumsulphate hitherto, silica gel, made by a complicated process whichinvolves the addition of sulphuric acid to potassium silicate solution,is formed with a slurry with a soluble vanadium compound, and the slurryis dried and heated to form the catalyst. Apart from the lengthy, costlyand troublesome character of this method of manufacture, the resultantcatalysts have the serious disadvantage that they are exceedinglyfriable and rapidly break down when used in a fluidized bed reactor andtherefore tend not to fluidize uniformly. In addition, a quantity ofcatalyst dust is formed in the space above the bed and this causes thetemperature above the bed to rise. Filters are normally provided inorder to prevent catalyst particles from escaping, and as thetemperature above the bed rises there is an increasing tendency for hotspots to form on the filters and damage them.

Our object is to produce improved catalyst particles, and we do this,according to the invention, by causing porous solid particles to absorbat least one material which is catalytic or becomes catalytic ondecomposition or other change in the course of or after the absorption.For simplicity of description, all such materials will be calledcatalytic materials herein. The process of absorption of the catalyticmaterial by the solid particles is carried out at a temperature at whichthe catalytic material melts. In carrying out our invention, we form afluidized bed of the porous particles and particles of the catalyticmaterial and we maintain the bed fluidized at the required temperatureuntil the catalytic material is taken up by the porous particles.

The invention can be applied to the manufacture of catalysts for use invarious different reactions. The necessary characteristic of thecatalytic material is that it must be capable of being introduced in theform of solid 3,352,887 Patented Nov. 14, 1967 particles into thefluidized bed of porous particles, and the solid particles of catalyticmaterial must melt at the temperature of the bed and wet the porousparticles so that the resultant liquid spreads out over and is absorbed0r adsorbed by the porous particles, which are in effect carriers of thecatalytic material.

It is important that the catalytic material should be wholly taken up bythe carrier particles. The catalytic material should not form acontinuous thick' layer on the outer surface because the mass ofparticles might then bind together when used as a catalyst. It isnecessary, of course, to choose the porous carrier particles with properregard to both the catalytic material and the reaction in which thecatalyst is to be used.

The carrier particles may most conveniently be silica gel, but may be,for instance, activated alumina, pumice, zeolites, clays or metallicoxides, provided that they are porous and stable at the fluidizingtemperature. However, not every silica gel is suitable as a carrier inevery catalytic reaction. An empirical test is to mix the silica gel orother carrier particles with the catalytic material in the desiredproportions and heat the mixture in a muflle furnace at a temperatureabove the melting point of the catalytic material with intermittentstirring. After a few hours, suitable carrier particles will give a drymobile powder, whereas unsuitable carrier particles will become a stickyagglomerated mass.

The degree of porosity of the carrier particles may vary considerably.In any case, the carrier particles must not sinter together at thetemperatures at which the catalyst is to be used.

We find that the smaller the size of the individual particles of thecarrier, the greater the proportional amount of the catalytic materialthey will take up. The smaller particles become completely impregnatedfirst, i.e. in a given time the amount of the catalytic material takenup is inversely proportional to the radius of the particle. This resultis advantageous when the catalyst is fluidized in reacting gases becausethe smaller particles become heavier and therefore less liable to beblown out of the fluidized bed. Typical particle size grading for silicagel are 50 to 200 13.8. (British Standard) sieves.

The means by which the objects of the invention are obtained aredescribed more fully with reference to the accompanying drawings, inwhich:

FIGURE 1 is a graph of the softening point of one form of catalyticmaterial;

FIGURE 2 is a graph of the heat of wetting of two different silica gels;and

FIGURE 3 is a diagrammatic view of an apparatus for carrying out theprocess of this invention.

The invention is particularly applicable to catalysts containingvanadium and potassium sulphate for use in the manufacture of phthalicanhydride. A mixture of vanadium pentoxide and potassium pyrosulphatewhen fused yields a glass which can be ground to fine particles andwhich has no sharp melting point. The softening point of such a mixturedepends on the molar ratio of the potassium pyrosulphate to the vanadiumpentoxide. This is illustrated by FIGURE 1 of the accompanying drawings,which is a graph of the molar ratio as K O:V O plotted as abscissaeagainst the softening point of resultant glass (measured by the ball andring method) plotted as ordinates. As may be seen from the graph thesoftening point is at a minimum when the molar ratio is about 8.5. Themolar ratio in the catalyst is of importance in determining itsreactivity, and is preferably within the range 1.0 to 6.0. Within thisrange, the softening point is below 320 C., so if the temperature in thefluidized bed is from 350 to 400 C. the glass will readily spread as aliquid over the surfaces of the carrier particles. The final activationof the heat of wetting in calories per gram is plotted as ordinates 7against the time in minutes plotted as abscissae. The curve marked Ishows a high heat of wetting for the first silica gel indicating that ithas a large internal surface area. The curve marked II shows a low heatof wetting extending over a long period and indicates that the secondsilica gel .II has a smaller pore diameter with a larger internalsurface area than the first. Both these gels have high internal specificsurfaces as measured by nitrogen absorption (500 sq. m./g. for the firstgel and 710 sq. m./g. for the second), but the pore diameter of thefirst gel (53 A.) is much larger than that of the second (14 A.). Thefirst gel is suitable for use in a catalyst to be used in the productionof phthalic anhydride, but the second is not. Broadly, it

may be said that for this purposea high heat of wetting and a pore sizeof at least 40 A. are required.

The ratio of catalytic material to carrier particles can vary widely.For example, with silicagel and the glass of vanadium and potassiumsulphate, the glass may be from 1 to 35% by weight of the catalyst. Verysatisfactory results are obtained with 25% by weight of a glass of molarratio K O:V O of 4:1. This gives a lower concentration.

of V in the final catalyst, about 4%, than the 9 to in the catalystsmade by the method usual hitherto. As the catalyst is as effective asthe prior catalysts there is thus a saving of vanadium. In addition, thenovel catalyst does not break down so readily.

One process according to the invention will now be described withreference to FIGURE 3 of the accompanying drawings, which showsdiagrammatically a plant in which the process may be'carried out.

The process described is for the manufacture of a catalystfor use in theproduction of phthalic anhydride. First a mixture of vanadium pentoxideand potassium pyrosulphate is melted, cast, ground to about 100 B.S.sieve mesh, weighed and mixed in a tumbling mill with some silica gelparticles of to 60 A. pore size and of particle size between 50' and 200B.S. sieve mesh.

The mixture is charged into a closed hopper 1 from which it can passunder the control of a valve 2 down a pipe 3 into a fiuidizing column 4.Compressed air sup-.

plied through a pipe 5 flows through a preheater 6, which is externallyheated electrically, and a pipe 7 to the base of the column 4 under thecontrol ofa valve 8 and emerges through a distributing ring 9. This airflows upwards through the column to maintain the particles fluidized inthe column. The temperature in the column is maintained between 350 and400" C. by external heating elements 10. Atthe top of the column thereare filters 11, through which the air passes to flow to a. chimneythrough pipes 12 and 13.

At the base of the column there is an outlet 14 controlled by a valve15, which is opened when the production of the catalyst is completewhile the flow of air is maintained so that the catalyst is dischargedin fluidized form, to be collected in drums.

For ease of delivery of the material from the hopper 1 to the column, 4,the particles in the hopper are occasionally fluidized. The air streamused to do this may be bled off from the. main air stream which flows upthe column. This air leaves the hopper through a pipe 16 and passes tothe filters 11, thus avoiding the provision of separate filterson thehopper.

It is possible for the main air flow from the pipe 5 to by-pass theheater 6 through a pipe 17 controlled by a valve 18, so that cold airmay be blown through the column 4 when it is empty to clear it of dust.

As an example, one column 4 was 23 feet high and 14 inches in diameter,and in it 480 pounds of silica gel'were fluidized by air at from 350 to400 C., the air velocity being /z-foot per second in the empty column.160 pounds of the powdered vanadium pentoxide potassium pyrosulphateglass mixed with 108 pounds of silica gel were added over a period of 3hours. The fluidization was maintained for a total of 8 hours, duringthe final 2 hours of which, the temperature was increased to about 420C. to activate the catalyst.

The pore structure of silica gel tends to shrink and collapse at about600 C. so the temperature in the column must be well below this figureand advantageously below 500 C. when the carrier particles are ofsilicagel.

The fluidized bed technique employed in the apparatus illustrated isextremely advantageous, being simple, leading to very rapid and perfectmixing, enabling the temperature to be closely and exactly controlledand involving plant of low capital cost having a high rate ofproduction.

Having now described the means by which the objects of the invention areobtained,

We claim:

A method of producing phthalic anhydride which comprises passing amixture of naphthalene vaporand air in contact with a fluidized bedcontaining a catalyst comprising particles of silica gel having a poresize of from about 40 to A. and a maximum sieve size of about 50 B.S.carrying a liquid deposited glass composed of a mixture of vanadiumpentoxide and potassium pyrosulphate having a molar ratio of K 0 :V Obetween 1.0 and 6.0 and a softening temperature below 320 C., andsaidglass amounting to from about 1 to 35 percent by weight of the totalmass of particles.

References Cited UNITED STATES PATENTS 2/1961 Chomitz et al. 260346.412/1965 Riley et a1. 2-52--440

